Instrument Detection with an Optical Touch Sensitive Device

ABSTRACT

An optical touch-sensitive device detects touch events caused by instruments (e.g., pens, styluses) and distinguishes these events from touch events caused by fingers. In some embodiments, different instruments can also be distinguished. The optical touch-sensitive device includes multiple emitters and detectors. Each emitter produces optical beams which are received by the detectors. The optical beams preferably are multiplexed in a manner so that many optical beams can be received by a detector simultaneously. Touch events disturb the optical beams, for example due to frustrated total internal reflection. Information indicating which optical beams have been disturbed is analyzed to detect one or more touch events. The analysis also distinguishes instrument touch events from finger touch events.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority under 35 U.S.C. §119(e) to U.S.Provisional Patent Application Ser. No. 62/044,875, “Pen Detection withan Optical Touch Sensitive Device title,” filed Sep. 2, 2014. Thesubject matter of all of the foregoing is incorporated herein byreference in their entirety.

BACKGROUND

1. Field of Art

This invention generally relates to detecting touch events in atouch-sensitive device.

2. Description of the Related Art

Touch-sensitive displays for interacting with computing devices arebecoming more common. A number of different technologies exist forimplementing touch-sensitive displays and other touch-sensitive devices.Examples of these techniques include, for example, resistive touchscreens, surface acoustic wave touch screens, capacitive touch screensand certain types of optical touch screens.

However, many of these approaches currently suffer from drawbacks. Forexample, some technologies may function well for small sized displays,as used in many modern mobile phones, but do not scale well to largerscreen sizes as in displays used with laptop or even desktop computers.For technologies that require a specially processed surface or the useof special elements in the surface, increasing the screen size by alinear factor of N means that the special processing must be scaled tohandle the N² larger area of the screen or that N² times as many specialelements are required. This can result in unacceptably low yields orprohibitively high costs.

Another drawback for some technologies is their inability or difficultyin handling multitouch events. A multitouch event occurs when multipletouch events occur simultaneously. This can introduce ambiguities in theraw detected signals, which then must be resolved. Importantly, theambiguities must be resolved in a speedy and computationally efficientmanner. If too slow, then the technology will not be able to deliver thetouch sampling rate required by the system. If too computationallyintensive, then this will drive up the cost and power consumption of thetechnology.

Another drawback is that technologies may not be able to meet increasingresolution demands. Assume that the touch-sensitive surface isrectangular with length and width dimensions L×W. Further assume that anapplication requires that touch points be located with an accuracy of δland δw, respectively. The effective required resolution is then R=(LW)/(δl δw). We will express R as the effective number of touch points.As technology progresses, the numerator in R generally will increase andthe denominator generally will decrease, thus leading to an overallincreasing trend for the required touch resolution R.

Thus, there is a need for improved touch-sensitive systems.

SUMMARY

An optical touch-sensitive device detects touch events caused byinstruments (e.g., pens, styluses) and distinguishes these events fromtouch events caused by fingers. In some embodiments, differentinstruments can also be distinguished.

The optical touch-sensitive device includes multiple emitters anddetectors. Each emitter produces optical beams which are received by thedetectors. The optical beams preferably are multiplexed in a manner sothat many optical beams can be received by a detector simultaneously.Touch events disturb the optical beams, for example due to frustratedtotal internal reflection. Information indicating which optical beamshave been disturbed is analyzed to detect one or more touch events. Theanalysis also distinguishes instrument touch events from finger touchevents.

Instruments can be distinguished from fingers on many different bases.One example is contact area. This can include size, shape and asymmetryof the contact area. The contact area for instruments can also bedesigned to include multiple disjoint regions. Another example isattenuation rates. Instruments can be constructed from materials whichwill exhibit a higher attenuation rate than fingers. Temporal behaviorcan also be used. A finger contacting a surface typically has adifferent temporal aspect than an instrument contacting a surface. Theactual instrument response, with respect to attenuating or enhancingoptical beams, can also be engineered to be different than that causedby fingers. Because instruments are manufactured, a much larger varietyof responses can be implemented, including redirecting incoming opticalbeams to different directions and splitting incoming optical beams intomultiple outgoing optical beams. Wavelength is yet another degree offreedom that can be used to distinguish instruments, both from fingersand from other instruments.

Active instruments can include the use of emitters and detectors.Emitters can inject additional optical beams into the system. Theseadditional optical beams can be used to detect the presence of theinstrument. They can also be designed to identify the instrument. Theycan also be used as a separate communication channel from theinstrument. Detectors can be used in the reverse direction. Opticalbeams created by emitters on the periphery can be detected and this canbe used to detect the presence of the instrument. Detected optical beamscan also be used as a communication channel to the instrument. Someinstruments may also have additional out of band communications, such asthrough a wireless channel.

Other modalities may also be used to detect instrument touch events.Examples include palm touches and acoustics. Since an instrument is heldin the user's hand, an instrument touch event is often accompanied by apalm touch in the vicinity. This can be used to help identify instrumenttouch events. Acoustic or vibration information can also be used todistinguish instrument touch events from finger touch events, due totheir different acoustic and vibration signatures.

Other aspects include components, devices, systems, improvements,methods, processes, applications, computer readable mediums, and othertechnologies related to any of the above.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments of the present invention will now be described, by way ofexample, with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of an optical touch-sensitive device, according toone embodiment.

FIG. 2 is a flow diagram for determining the locations of touch events,according to one embodiment.

FIGS. 3A-3F illustrate different mechanisms for a touch interaction withan optical beam.

FIG. 4 are graphs of binary and analog touch interactions.

FIGS. 5A-5C are top views of differently shaped beam footprints.

FIGS. 6A-6B are top views illustrating a touch point travelling througha narrow beam and a wide beam, respectively.

FIG. 7 are graphs of the binary and analog responses for the narrow andwide beams of FIG. 6.

FIGS. 8A-8B are top views illustrating active area coverage by emitters.

FIG. 8C is a top view illustrating alternating emitters and detectors.

FIGS. 9A-9E are top views of different types of two-dimensional contactareas for instruments.

FIG. 10A illustrates a finger touch progressing in time.

FIGS. 10B and 10C illustrate instrument touches progressing in time.

FIG. 11 is a diagram of nomenclature used to define an instrumentresponse.

FIGS. 12A-12B are a front view and a side cross-sectional view of a tipstructure using internal waveguide channels.

FIG. 12C illustrates operation of the tip structure in FIGS. 12A-12B.

FIG. 13 is a diagram of a tip structure that redirects light.

FIG. 14 is a diagram of a wavelength-selective tip structure.

FIG. 15 is a diagram of a tip structure using a grating.

FIG. 16 is a diagram of a tip structure with an intermediate index ofrefraction.

FIG. 17A is a side view of an injector tip.

FIGS. 17B-17D are top view of different types of injector tips.

FIG. 18 is a flow diagram for qualifying possible instrument touches.

DETAILED DESCRIPTION

This detailed description is divided into two parts. Part A provides adescription of various aspects of touch-sensitive systems and thedetection of multitouch events. These are described in the context offinger touches, but the concepts apply also to instrument (e.g., pen orstylus) touches. Part B provides a description of detecting instrumenttouches, including distinguishing between different types ofinstruments. The following is the contents of the detailed description:

Part A: Touch Detection

I. Introduction

II. Physical Set-up

III. Processing Phase

Part B: Instrument Detection

IV. Introduction

V. Passive Instrument Detection

VI. Active Instrument Detection

VII. Additional Modalities

Part A: Touch Detection I. Introduction

A. Device Overview

FIG. 1 is a diagram of an optical touch-sensitive device 100, accordingto one embodiment. The optical touch-sensitive device 100 includes acontroller 110, emitter/detector drive circuits 120, and atouch-sensitive surface assembly 130. The surface assembly 130 includesa surface 131 over which touch events are to be detected. Forconvenience, the area defined by surface 131 may sometimes be referredto as the active area or active surface, even though the surface itselfmay be an entirely passive structure. The assembly 130 also includesemitters and detectors arranged along the periphery of the activesurface 131. In this example, there are J emitters labeled as Ea-EJ andK detectors labeled as D1-DK. The device also includes a touch eventprocessor 140, which may be implemented as part of the controller 110 orseparately as shown in FIG. 1. A standardized API may be used tocommunicate with the touch event processor 140, for example between thetouch event processor 140 and controller 110, or between the touch eventprocessor 140 and other devices connected to the touch event processor.

The emitter/detector drive circuits 120 serve as an interface betweenthe controller 110 and the emitters Ej and detectors Dk. The emittersproduce optical “beams” which are received by the detectors. Preferably,the light produced by one emitter is received by more than one detector,and each detector receives light from more than one emitter. Forconvenience, “beam” will refer to the light from one emitter to onedetector, even though it may be part of a large fan of light that goesto many detectors rather than a separate beam. The beam from emitter Ejto detector Dk will be referred to as beam jk. FIG. 1 expressly labelsbeams a1, a2, a3, e1 and eK as examples. Touches within the active area131 will disturb certain beams, thus changing what is received at thedetectors Dk. Data about these changes is communicated to the touchevent processor 140, which analyzes the data to determine thelocation(s) (and times) of touch events on surface 131.

One advantage of an optical approach as shown in FIG. 1 is that thisapproach scales well to larger screen sizes. Since the emitters anddetectors are positioned around the periphery, increasing the screensize by a linear factor of N means that the periphery also scales by afactor of N rather than N².

These touch-sensitive devices can be used in various applications.Touch-sensitive displays are one class of application. This includesdisplays for tablets, laptops, desktops, gaming consoles, smart phonesand other types of compute devices. It also includes displays for TVs,digital signage, public information, whiteboards, e-readers and othertypes of good resolution displays. However, they can also be used onsmaller or lower resolution displays: simpler cell phones, user controls(photocopier controls, printer controls, control of appliances, etc.).These touch-sensitive devices can also be used in applications otherthan displays. The “surface” over which the touches are detected couldbe a passive element, such as a printed image or simply some hardsurface. This application could be used as a user interface, similar toa trackball or mouse.

B. Process Overview

FIG. 2 is a flow diagram for determining the locations of touch events,according to one embodiment. This process will be illustrated using thedevice of FIG. 1. The process 200 is roughly divided into two phases,which will be referred to as a physical phase 210 and a processing phase220. Conceptually, the dividing line between the two phases is a set oftransmission coefficients Tjk.

The transmission coefficient Tjk is the transmittance of the opticalbeam from emitter j to detector k, compared to what would have beentransmitted if there was no touch event interacting with the opticalbeam. In the following examples, we will use a scale of 0 (fully blockedbeam) to 1 (fully transmitted beam). Thus, a beam jk that is undisturbedby a touch event has Tjk=1. A beam jk that is fully blocked by a touchevent has a Tjk=0. A beam jk that is partially blocked or attenuated bya touch event has 0<Tjk<1. It is possible for Tjk>1, for exampledepending on the nature of the touch interaction or in cases where lightis deflected or scattered to detectors k that it normally would notreach.

The use of this specific measure is purely an example. Other measurescan be used. In particular, since we are most interested in interruptedbeams, an inverse measure such as (1−Tjk) may be used since it isnormally 0. Other examples include measures of absorption, attenuation,reflection or scattering. In addition, although FIG. 2 is explainedusing Tjk as the dividing line between the physical phase 210 and theprocessing phase 220, it is not required that Tjk be expresslycalculated. Nor is a clear division between the physical phase 210 andprocessing phase 220 required.

Returning to FIG. 2, the physical phase 210 is the process ofdetermining the Tjk from the physical setup. The processing phase 220determines the touch events from the Tjk. The model shown in FIG. 2 isconceptually useful because it somewhat separates the physical setup andunderlying physical mechanisms from the subsequent processing.

For example, the physical phase 210 produces transmission coefficientsTjk. Many different physical designs for the touch-sensitive surfaceassembly 130 are possible, and different design tradeoffs will beconsidered depending on the end application. For example, the emittersand detectors may be narrower or wider, narrower angle or wider angle,various wavelengths, various powers, coherent or not, etc. As anotherexample, different types of multiplexing may be used to allow beams frommultiple emitters to be received by each detector. Several of thesephysical setups and manners of operation are described below, primarilyin Section II.

The interior of block 210 shows one possible implementation of process210. In this example, emitters transmit 212 beams to multiple detectors.Some of the beams travelling across the touch-sensitive surface aredisturbed by touch events. The detectors receive 214 the beams from theemitters in a multiplexed optical form. The received beams arede-multiplexed 216 to distinguish individual beams jk from each other.Transmission coefficients Tjk for each individual beam jk are thendetermined 218.

The processing phase 220 can also be implemented in many different ways.Candidate touch points, line imaging, location interpolation, touchevent templates and multi-pass approaches are all examples of techniquesthat may be used as part of the processing phase 220. Several of theseare described below, primarily in Section III.

II. Physical Set-Up

The touch-sensitive device 100 may be implemented in a number ofdifferent ways. The following are some examples of design variations.

A. Electronics

With respect to electronic aspects, note that FIG. 1 is exemplary andfunctional in nature. Functions from different boxes in FIG. 1 can beimplemented together in the same component.

For example, the controller 110 and touch event processor 140 may beimplemented as hardware, software or a combination of the two. They mayalso be implemented together (e.g., as an SoC with code running on aprocessor in the SoC) or separately (e.g., the controller as part of anASIC, and the touch event processor as software running on a separateprocessor chip that communicates with the ASIC). Example implementationsinclude dedicated hardware (e.g., ASIC or programmed field programmablegate array (FPGA)), and microprocessor or microcontroller (eitherembedded or standalone) running software code (including firmware).Software implementations can be modified after manufacturing by updatingthe software.

The emitter/detector drive circuits 120 serve as an interface betweenthe controller 110 and the emitters and detectors. In oneimplementation, the interface to the controller 110 is at least partlydigital in nature. With respect to emitters, the controller 110 may sendcommands controlling the operation of the emitters. These commands maybe instructions, for example a sequence of bits which mean to takecertain actions: start/stop transmission of beams, change to a certainpattern or sequence of beams, adjust power, power up/power downcircuits. They may also be simpler signals, for example a “beam enablesignal,” where the emitters transmit beams when the beam enable signalis high and do not transmit when the beam enable signal is low.

The circuits 120 convert the received instructions into physical signalsthat drive the emitters. For example, circuit 120 might include somedigital logic coupled to digital to analog converters, in order toconvert received digital instructions into drive currents for theemitters. The circuit 120 might also include other circuitry used tooperate the emitters: modulators to impress electrical modulations ontothe optical beams (or onto the electrical signals driving the emitters),control loops and analog feedback from the emitters, for example. Theemitters may also send information to the controller, for exampleproviding signals that report on their current status.

With respect to the detectors, the controller 110 may also send commandscontrolling the operation of the detectors, and the detectors may returnsignals to the controller. The detectors also transmit information aboutthe beams received by the detectors. For example, the circuits 120 mayreceive raw or amplified analog signals from the detectors. The circuitsthen may condition these signals (e.g., noise suppression), convert themfrom analog to digital form, and perhaps also apply some digitalprocessing (e.g., demodulation).

B. Touch Interactions

FIGS. 3A-3F illustrate different mechanisms for a touch interaction withan optical beam. FIG. 3A illustrates a mechanism based on frustratedtotal internal reflection (TIR). The optical beam, shown as a dashedline, travels from emitter E to detector D through an opticallytransparent planar waveguide 302. The beam is confined to the waveguide302 by total internal reflection. The waveguide may be constructed ofplastic or glass, for example. An object 304, such as a finger orstylus, coming into contact with the transparent waveguide 302, has ahigher refractive index than the air normally surrounding the waveguide.Over the area of contact, the increase in the refractive index due tothe object disturbs the total internal reflection of the beam within thewaveguide. The disruption of total internal reflection increases thelight leakage from the waveguide, attenuating any beams passing throughthe contact area. Correspondingly, removal of the object 304 will stopthe attenuation of the beams passing through. Attenuation of the beamspassing through the touch point will result in less power at thedetectors, from which the reduced transmission coefficients Tjk can becalculated.

FIG. 3B illustrates a mechanism based on beam blockage. Emitters producebeams which are in close proximity to a surface 306. An object 304coming into contact with the surface 306 will partially or entirelyblock beams within the contact area. FIGS. 3A and 3B illustrate somephysical mechanisms for touch interactions, but other mechanisms canalso be used. For example, the touch interaction may be based on changesin polarization, scattering, or changes in propagation direction orpropagation angle (either vertically or horizontally).

For example, FIG. 3C illustrates a different mechanism based onpropagation angle. In this example, the optical beam is guided in awaveguide 302 via TIR. The optical beam hits the waveguide-air interfaceat a certain angle and is reflected back at the same angle. However, thetouch 304 changes the angle at which the optical beam is propagating. InFIG. 3C, the optical beam travels at a steeper angle of propagationafter the touch 304. The detector D has a response that varies as afunction of the angle of propagation. The detector D could be moresensitive to the optical beam travelling at the original angle ofpropagation or it could be less sensitive. Regardless, an optical beamthat is disturbed by a touch 304 will produce a different response atdetector D.

In FIGS. 3A-3C, the touching object was also the object that interactedwith the beam. This will be referred to as a direct interaction. In anindirect interaction, the touching object interacts with an intermediateobject, which interacts with the optical beam. FIG. 3D shows an examplethat uses intermediate blocking structures 308. Normally, thesestructures 308 do not block the beam. However, in FIG. 3D, object 304contacts the blocking structure 308, which causes it to partially orentirely block the optical beam. In FIG. 3D, the structures 308 areshown as discrete objects, but they do not have to be so.

In FIG. 3E, the intermediate structure 310 is a compressible, partiallytransmitting sheet. When there is no touch, the sheet attenuates thebeam by a certain amount. In FIG. 3E, the touch 304 compresses thesheet, thus changing the attenuation of the beam. For example, the upperpart of the sheet may be more opaque than the lower part, so thatcompression decreases the transmittance. Alternately, the sheet may havea certain density of scattering sites. Compression increases the densityin the contact area, since the same number of scattering sites occupiesa smaller volume, thus decreasing the transmittance. Analogous indirectapproaches can also be used for frustrated TIR. Note that this approachcould be used to measure contact pressure or touch velocity, based onthe degree or rate of compression.

The touch mechanism may also enhance transmission, instead of or inaddition to reducing transmission. For example, the touch interaction inFIG. 3E might increase the transmission instead of reducing it. Theupper part of the sheet may be more transparent than the lower part, sothat compression increases the transmittance.

FIG. 3F shows another example where the transmittance between an emitterand detector increases due to a touch interaction. FIG. 3F is a topview. Emitter Ea normally produces a beam that is received by detectorD1. When there is no touch interaction, Ta1=1 and Ta2=0. However, atouch interaction 304 blocks the beam from reaching detector D1 andscatters some of the blocked light to detector D2. Thus, detector D2receives more light from emitter Ea than it normally would. Accordingly,when there is a touch event 304, Ta1 decreases and Ta2 increases.

As will be described in detail in Part B, if the object 304 is aninstrument, the instrument can be designed to have certain touchinteraction characteristics. For example, the touch interaction causedby the instrument 304 may vary as a function of wavelength, or theinteraction may change as the instrument is tilted, translated, rotatedor otherwise moved. The touch interaction with the instrument 304 mayalso depend on the propagation direction of the optical beam. Theinstrument 304 may also be an active device, with its own emitter(s)and/or detector(s). It may also include re-emitter(s), which detectincoming optical beams and then re-emit the beams, possibly changing thebeams before re-emission.

For simplicity, in the remainder of this Part A, the touch mechanismwill be assumed to be primarily of a blocking nature, meaning that abeam from an emitter to a detector will be partially or fully blocked byan intervening touch event. This is not required, but it is convenientto illustrate various concepts.

For convenience, the touch interaction mechanism may sometimes beclassified as either binary or analog. A binary interaction is one thatbasically has two possible responses as a function of the touch.Examples includes non-blocking and fully blocking, or non-blocking and10%+attenuation, or not frustrated and frustrated TIR. An analoginteraction is one that has a “grayscale” response to the touch:non-blocking passing through gradations of partially blocking toblocking Whether the touch interaction mechanism is binary or analogdepends in part on the nature of the interaction between the touch andthe beam. It does not depend on the lateral width of the beam (which canalso be manipulated to obtain a binary or analog attenuation, asdescribed below), although it might depend on the vertical size of thebeam.

FIG. 4 is a graph illustrating a binary touch interaction mechanismcompared to an analog touch interaction mechanism. FIG. 4 graphs thetransmittance Tjk as a function of the depth z of the touch. Thedimension z is into and out of the active surface. Curve 410 is a binaryresponse. At low z (i.e., when the touch has not yet disturbed thebeam), the transmittance Tjk is at its maximum. However, at some pointz₀, the touch breaks the beam and the transmittance Tjk falls fairlysuddenly to its minimum value. Curve 420 shows an analog response wherethe transition from maximum Tjk to minimum Tjk occurs over a wider rangeof z. If curve 420 is well behaved, it is possible to estimate z fromthe measured value of Tjk.

C. Emitters, Detectors and Couplers

Each emitter transmits light to a number of detectors. Usually, eachemitter outputs light to more than one detector simultaneously.Similarly, each detector receives light from a number of differentemitters. The optical beams may be visible, infrared and/or ultravioletlight. The term “light” is meant to include all of these wavelengths andterms such as “optical” are to be interpreted accordingly.

Examples of the optical sources for the emitters include light emittingdiodes (LEDs) and semiconductor lasers. IR sources can also be used.Modulation of optical beams can be achieved by directly modulating theoptical source or by using an external modulator, for example a liquidcrystal modulator or a deflected mirror modulator. Examples of sensorelements for the detector include charge coupled devices, photodiodes,photoresistors, phototransistors, and nonlinear all-optical detectors.Typically, the detectors output an electrical signal that is a functionof the intensity of the received optical beam.

The emitters and detectors may also include optics and/or electronics inaddition to the main optical source and sensor element. For example,optics can be used to couple between the emitter/detector and thedesired beam path. Optics can also reshape or otherwise condition thebeam produced by the emitter or accepted by the detector. These opticsmay include lenses, Fresnel lenses, mirrors, filters, non-imaging opticsand other optical components.

In this disclosure, the optical paths will be shown unfolded forclarity. Thus, sources, optical beams and sensors will be shown as lyingin one plane. In actual implementations, the sources and sensorstypically will not lie in the same plane as the optical beams. Variouscoupling approaches can be used. A planar waveguide or optical fiber maybe used to couple light to/from the actual beam path. Free spacecoupling (e.g., lenses and mirrors) may also be used. A combination mayalso be used, for example waveguided along one dimension and free spacealong the other dimension. Various coupler designs are described in U.S.Application Ser. No. 61/510,989 “Optical Coupler” filed on Jul. 22,2011, which is incorporated by reference in its entirety herein.

D. Optical Beam Paths

Another aspect of a touch-sensitive system is the shape and location ofthe optical beams and beam paths. In FIGS. 1-2, the optical beams areshown as lines. These lines should be interpreted as representative ofthe beams, but the beams themselves are not necessarily narrow pencilbeams. FIGS. 5A-5C illustrate different beam shapes.

FIG. 5A shows a point emitter E, point detector D and a narrow “pencil”beam 510 from the emitter to the detector. In FIG. 5B, a point emitter Eproduces a fan-shaped beam 520 received by the wide detector D. In FIG.5C, a wide emitter E produces a “rectangular” beam 530 received by thewide detector D. These are top views of the beams and the shapes shownare the footprints of the beam paths. Thus, beam 510 has a line-likefootprint, beam 520 has a triangular footprint which is narrow at theemitter and wide at the detector, and beam 530 has a fairly constantwidth rectangular footprint. In FIG. 5, the detectors and emitters arerepresented by their widths, as seen by the beam path. The actualoptical sources and sensors may not be so wide. Rather, optics (e.g.,cylindrical lenses or mirrors) can be used to effectively widen ornarrow the lateral extent of the actual sources and sensors.

FIGS. 6A-6B and 7 show how the width of the footprint can determinewhether the transmission coefficient Tjk behaves as a binary or analogquantity. In these figures, a touch point has contact area 610. Assumethat the touch is fully blocking, so that any light that hits contactarea 610 will be blocked. FIG. 6A shows what happens as the touch pointmoves left to right past a narrow beam. In the leftmost situation, thebeam is not blocked at all (i.e., maximum Tjk) until the right edge ofthe contact area 610 interrupts the beam. At this point, the beam isfully blocked (i.e., minimum Tjk), as is also the case in the middlescenario. It continues as fully blocked until the entire contact areamoves through the beam. Then, the beam is again fully unblocked, asshown in the righthand scenario. Curve 710 in FIG. 7 shows thetransmittance Tjk as a function of the lateral position x of the contactarea 610. The sharp transitions between minimum and maximum Tjk show thebinary nature of this response.

FIG. 6B shows what happens as the touch point moves left to right past awide beam. In the leftmost scenario, the beam is just starting to beblocked. The transmittance Tjk starts to fall off but is at some valuebetween the minimum and maximum values. The transmittance Tjk continuesto fall as the touch point blocks more of the beam, until the middlesituation where the beam is fully blocked. Then the transmittance Tjkstarts to increase again as the contact area exits the beam, as shown inthe righthand situation. Curve 720 in FIG. 7 shows the transmittance Tjkas a function of the lateral position x of the contact area 610. Thetransition over a broad range of x shows the analog nature of thisresponse.

FIGS. 5-7 consider an individual beam path. In most implementations,each emitter and each detector will support multiple beam paths.

FIG. 8A is a top view illustrating the beam pattern produced by a pointemitter. Emitter Ej transmits beams to wide detectors D1-DK. Three beamsare shaded for clarity: beam j1, beam j(K−1) and an intermediate beam.Each beam has a fan-shaped footprint. The aggregate of all footprints isemitter Ej's coverage area. That is, any touch event that falls withinemitter Ej's coverage area will disturb at least one of the beams fromemitter Ej. FIG. 8B is a similar diagram, except that emitter Ej is awide emitter and produces beams with “rectangular” footprints (actually,trapezoidal but we will refer to them as rectangular). The three shadedbeams are for the same detectors as in FIG. 8A.

Note that every emitter Ej may not produce beams for every detector Dk.In FIG. 1, consider beam path aK which would go from emitter Ea todetector DK. First, the light produced by emitter Ea may not travel inthis direction (i.e., the radiant angle of the emitter may not be wideenough) so there may be no physical beam at all, or the acceptance angleof the detector may not be wide enough so that the detector does notdetect the incident light. Second, even if there was a beam and it wasdetectable, it may be ignored because the beam path is not located in aposition to produce useful information. Hence, the transmissioncoefficients Tjk may not have values for all combinations of emitters Ejand detectors Dk.

The footprints of individual beams from an emitter and the coverage areaof all beams from an emitter can be described using differentquantities. Spatial extent (i.e., width), angular extent (i.e., radiantangle for emitters, acceptance angle for detectors) and footprint shapeare quantities that can be used to describe individual beam paths aswell as an individual emitter's coverage area.

An individual beam path from one emitter Ej to one detector Dk can bedescribed by the emitter Ej's width, the detector Dk's width and/or theangles and shape defining the beam path between the two.

These individual beam paths can be aggregated over all detectors for oneemitter Ej to produce the coverage area for emitter Ej. Emitter Ej'scoverage area can be described by the emitter Ej's width, the aggregatewidth of the relevant detectors Dk and/or the angles and shape definingthe aggregate of the beam paths from emitter Ej. Note that theindividual footprints may overlap (see FIG. 8B close to the emitter).Therefore an emitter's coverage area may not be equal to the sum of itsfootprints. The ratio of (the sum of an emitter's footprints)/(emitter'scover area) is one measure of the amount of overlap.

The coverage areas for individual emitters can be aggregated over allemitters to obtain the overall coverage for the system. In this case,the shape of the overall coverage area is not so interesting because itshould cover the entirety of the active area 131. However, not allpoints within the active area 131 will be covered equally. Some pointsmay be traversed by many beam paths while other points traversed by farfewer. The distribution of beam paths over the active area 131 may becharacterized by calculating how many beam paths traverse different(x,y) points within the active area. The orientation of beam paths isanother aspect of the distribution. An (x,y) point that is derived fromthree beam paths that are all running roughly in the same directionusually will be a weaker distribution than a point that is traversed bythree beam paths that all run at 60 degree angles to each other.

The discussion above for emitters also holds for detectors. The diagramsconstructed for emitters in FIGS. 8A-8B can also be constructed fordetectors. A detector Dk's coverage area is then the aggregate of allfootprints for beams received by a detector Dk. The aggregate of alldetector coverage areas gives the overall system coverage.

E. Active Area Coverage

The coverage of the active area 131 depends on the shapes of the beampaths, but also depends on the arrangement of emitters and detectors. Inmost applications, the active area is rectangular in shape, and theemitters and detectors are located along the four edges of therectangle.

In a preferred approach, rather than having only emitters along certainedges and only detectors along the other edges, emitters and detectorsare interleaved along the edges. FIG. 8C shows an example of this whereemitters and detectors are alternated along all four edges. The shadedbeams show the coverage area for emitter Ej.

F. Multiplexing

Since multiple emitters transmit multiple optical beams to multipledetectors, and since the behavior of individual beams is generallydesired, a multiplexing/demultiplexing scheme is used. For example, eachdetector typically outputs a single electrical signal indicative of theintensity of the incident light, regardless of whether that light isfrom one optical beam produced by one emitter or from many optical beamsproduced by many emitters. However, the transmittance Tjk is acharacteristic of an individual optical beam jk.

Different types of multiplexing can be used. Depending upon themultiplexing scheme used, the transmission characteristics of beams,including their content and when they are transmitted, may vary.Consequently, the choice of multiplexing scheme may affect both thephysical construction of the optical touch-sensitive device as well asits operation.

One approach is based on code division multiplexing. In this approach,the optical beams produced by each emitter are encoded using differentcodes. A detector receives an optical signal which is the combination ofoptical beams from different emitters, but the received beam can beseparated into its components based on the codes. This is described infurther detail in U.S. application Ser. No. 13/059,772 “Optical ControlSystem With Modulated Emitters,” which is incorporated by referenceherein.

Another similar approach is frequency division multiplexing. In thisapproach, rather than modulated by different codes, the optical beamsfrom different emitters are modulated by different frequencies. Thefrequencies are low enough that the different components in the detectedoptical beam can be recovered by electronic filtering or otherelectronic or software means.

Time division multiplexing can also be used. In this approach, differentemitters transmit beams at different times. The optical beams andtransmission coefficients Tjk are identified based on timing. If onlytime multiplexing is used, the controller must cycle through theemitters quickly enough to meet the required touch sampling rate.

Other multiplexing techniques commonly used with optical systems includewavelength division multiplexing, polarization multiplexing, spatialmultiplexing and angle multiplexing. Electronic modulation schemes, suchas PSK, QAM and OFDM, may also be possibly applied to distinguishdifferent beams.

Several multiplexing techniques may be used together. For example, timedivision multiplexing and code division multiplexing could be combined.Rather than code division multiplexing 128 emitters or time divisionmultiplexing 128 emitters, the emitters might be broken down into 8groups of 16. The 8 groups are time division multiplexed so that only 16emitters are operating at any one time, and those 16 emitters are codedivision multiplexed. This might be advantageous, for example, tominimize the number of emitters active at any given point in time toreduce the power requirements of the device.

III. Processing Phase

In the processing phase 220 of FIG. 2, the transmission coefficients Tjkare used to determine the locations of touch points. Differentapproaches and techniques can be used, including candidate touch points,line imaging, location interpolation, touch event templates, multi-passprocessing and beam weighting.

A. Candidate Touch Points

One approach to determine the location of touch points is based onidentifying beams that have been affected by a touch event (based on thetransmission coefficients Tjk) and then identifying intersections ofthese interrupted beams as candidate touch points. The list of candidatetouch points can be refined by considering other beams that are inproximity to the candidate touch points or by considering othercandidate touch points. This approach is described in further detail inU.S. patent application Ser. No. 13/059,817, “Method and Apparatus forDetecting a Multitouch Event in an Optical Touch-Sensitive Device,”which is incorporated herein by reference.

B. Line Imaging, Tomography

This technique is based on the concept that the set of beams received bya detector form a line image of the touch points, where the viewpoint isthe detector's location. The detector functions as a one-dimensionalcamera that is looking at the collection of emitters. Due toreciprocity, the same is also true for emitters. The set of beamstransmitted by an emitter form a line image of the touch points, wherethe viewpoint is the emitter's location. These line images can beprocessed to reconstruct the touch points, for example by usingcorrelation or tomography principles. This approach is described infurther detail in U.S. patent application Ser. No. 13/460,703,“Detecting Multitouch Events in an Optical Touch-Sensitive Device usingTouch Event Templates,” and Ser. No. 14/092,850, “Optical TouchTomography,” which are incorporated herein by reference.

C. Location Interpolation

Applications typically will require a certain level of accuracy inlocating touch points. One approach to increase accuracy is to increasethe density of emitters, detectors and beam paths so that a small changein the location of the touch point will interrupt different beams.Another approach is to interpolate between beams. This approach isdescribed in further detail in U.S. patent application Ser. No.13/460,703, “Detecting Multitouch Events in an Optical Touch-SensitiveDevice using Touch Event Templates,” which is incorporated herein byreference.

D. Touch Event Templates

If the locations and shapes of the beam paths are known, which istypically the case for systems with fixed emitters, detectors andoptics, it is possible to predict in advance the transmissioncoefficients for a given touch event. Templates can be generated apriori for expected touch events. The determination of touch events thenbecomes a template matching problem.

If a brute force approach is used, then one template can be generatedfor each possible touch event. However, this can result in a largenumber of templates. For example, assume that one class of touch eventsis modeled as oval contact areas and assume that the beams are pencilbeams that are either fully blocked or fully unblocked. This class oftouch events can be parameterized as a function of five dimensions:length of major axis, length of minor axis, orientation of major axis, xlocation within the active area and y location within the active area. Abrute force exhaustive set of templates covering this class of touchevents must span these five dimensions. In addition, the template itselfmay have a large number of elements.

Thus, in another approach, the set of templates is simplified. Forexample, one possible template for a touch event with a certain contactarea is the set of all beam paths that would be affected by the touch.However, this is a large number of beam paths, so template matching willbe more difficult. In addition, this template is very specific tocontact area. If the contact area changes slightly in size, shape orposition, the template for contact area will no longer match exactly.Also, if additional touches are present elsewhere in the active area,the template will not match the detected data well. Thus, although usingall possible beam paths can produce a fairly discriminating template, itcan also be computationally intensive to implement. An alternative usestemplates with less than all affected beams. For example, a simplertemplate may be based on only four beams that would be interrupted by acertain contact area. This is a less specific template since othercontact areas of slightly different shape, size or location will stillmatch this template. This is good in the sense that fewer templates willbe required to cover the space of possible contact areas. This templateis less precise than the full template based on all interrupted beams.However, it is also faster to match due to the smaller size. These typesof templates often are sparse relative to the full set of possibletransmission coefficients.

Note that a series of templates could be defined for a certain contactarea, increasing in the number of beams contained in the template: a2-beam template, a 4-beam template, etc. In one embodiment, the beamsthat are interrupted by contact area are ordered sequentially from 1 toN. An n-beam template can then be constructed by selecting the first nbeams in the order. Generally speaking, beams that are spatially orangularly diverse tend to yield better templates. That is, a templatewith three beams running at 60 degrees to each other and notintersecting at a common point tends to produce a more robust templatethan one based on three largely parallel beams which are in closeproximity to each other. In addition, more beams tends to increase theeffective signal-to-noise ratio of the template matching, particularlyif the beams are from different emitters and detectors.

Often, a base template can also be used to generate a family of similartemplates. For example, contact area B may be is the same as contactarea A, but shifted to the right. The corresponding four-beam templatefor contact area B can then be generated from the template for contactarea A, by making use of the right shift. More generally, the templatefor contact area A can be abstracted or parameterized (e.g., where theparameters are the amount of shift in different directions). Theabstraction will be referred to as a template model. In one approach,the model is used to generate the individual templates and the actualdata is matched against each of the individual templates. In anotherapproach, the data is matched against the template model. The matchingprocess then includes determining whether there is a match against thetemplate model and, if so, which value of the parameters produces thematch.

Templates can use both positive and negative regions. An actual contactarea may be surrounded by a “touch-free” zone. If contact is made in theactual contact area, then there will be no contact in the immediatelysurrounding area. Thus, the template includes both (a) beams in thecontact area that are interrupted, and (b) beams in the shaded area thatare not interrupted.

Templates can also be based both on reduced and enhanced transmissioncoefficients. For a particular type of contact, the transmissioncoefficients for certain beams that are interrupted should decrease.However, the touch interaction may scatter or reflect light in otherdirections, and the transmission coefficients for these directionsshould increase.

Other templates will be apparent and templates can be processed in anumber of ways. In a straightforward approach, the disturbances for thebeams in a template are simply summed or averaged. This can increase theoverall SNR for such a measurement, because each beam adds additionalsignal while the noise from each beam is presumably independent. Inanother approach, the sum or other combination could be a weightedprocess, where not all beams in the template are given equal weight. Forexample, the beams which pass close to the center of the touch eventbeing modeled could be weighted more heavily than those that are furtheraway. Alternately, the angular diversity of beams in the template couldalso be expressed by weighting. Angular diverse beams are more heavilyweighted than beams that are not as diverse.

Additional examples of touch event templates are described in furtherdetail in U.S. patent application Ser. No. 13/460,703, “DetectingMultitouch Events in an Optical Touch-Sensitive Device using Touch EventTemplates,” which is incorporated herein by reference.

E. Multi-Pass Processing

Referring to FIG. 2, the processing phase need not be a single-passprocess nor is it limited to a single technique. Multiple processingtechniques may be combined or otherwise used together to determine thelocations of touch events.

As one example, a first stage is a coarse pass that relies on a fastbinary template matching. In this stage, the templates are binary andthe transmittances T′jk are also assumed to be binary. The binarytransmittances T′jk can be generated from the analog values Tjk byrounding or thresholding the analog values. The binary values T′jk arematched against binary templates to produce a preliminary list ofcandidate touch points. Some clean-up is performed to refine this list.For example, it may be simple to eliminate redundant candidate touchpoints or to combine candidate touch points that are close or similar toeach other. A second stage is used to eliminate false positives, using amore refined approach. For each candidate touch point, neighboring beamsmay be used to validate or eliminate the candidate as an actual touchpoint. The techniques described in U.S. patent application Ser. No.13/059,817 may be used for this purpose. This stage may also use theanalog values Tjk, in addition to accounting for the actual width of theoptical beams. The output of stage is a list of confirmed touch points.The final stage refines the location of each touch point. For example,the interpolation techniques described previously can be used todetermine the locations with better accuracy. Since the approximatelocation is already known, stage may work with a much smaller number ofbeams (i.e., those in the local vicinity) but might apply more intensivecomputations to that data. The end result is a determination of thetouch locations.

Other techniques may also be used for multi-pass processing. Forexample, line images or touch event models may also be used.Alternatively, the same technique may be used more than once or in aniterative fashion. For example, low resolution templates may be usedfirst to determine a set of candidate touch locations, and then higherresolution templates or touch event models may be used to more preciselydetermine the precise location and shape of the touch.

F. Beam Weighting

In processing the transmission coefficients, it is common to weight orto prioritize the transmission coefficients. Weighting effectively meansthat some beams are more important than others. Weightings may bedetermined during processing as needed, or they may be predetermined andretrieved from lookup tables or lists.

One factor for weighting beams is angular diversity. Usually, angularlydiverse beams are given a higher weight than beams with comparativelyless angular diversity. Given one beam, a second beam with small angulardiversity (i.e., roughly parallel to the first beam) may be weightedlower because it provides relatively little additional information aboutthe location of the touch event beyond what the first beam provides.Conversely, a second beam which has a high angular diversity relative tothe first beam may be given a higher weight in determining where alongthe first beam the touch point occurs.

Another factor for weighting beams is position difference between theemitters and/or detectors of the beams (i.e., spatial diversity).Usually, greater spatial diversity is given a higher weight since itrepresents “more” information compared to what is already available.

Another possible factor for weighting beams is the density of beams. Ifthere are many beams traversing a region of the active area, then eachbeam is just one of many and any individual beam is less important andmay be weighted less. Conversely, if there are few beams traversing aregion of the active area, then each of those beams is more significantin the information that it carries and may be weighted more.

In another aspect, the nominal beam transmittance (i.e., thetransmittance in the absence of a touch event) could be used to weightbeams. Beams with higher nominal transmittance can be considered to bemore “trustworthy” than those which have lower nominal transmittancesince those are more vulnerable to noise. A signal-to-noise ratio, ifavailable, can be used in a similar fashion to weight beams. Beams withhigher signal-to-noise ratio may be considered to be more “trustworthy”and given higher weight.

The weightings, however determined, can be used in the calculation of afigure of merit (confidence) of a given template associated with apossible touch location. Beam transmittance/signal-to-noise ratio canalso be used in the interpolation process, being gathered into a singlemeasurement of confidence associated with the interpolated line derivedfrom a given touch shadow in a line image. Those interpolated lineswhich are derived from a shadow composed of “trustworthy” beams can begiven greater weight in the determination of the final touch pointlocation than those which are derived from dubious beam data.

Part B: Instrument Detection IV. Introduction

Detection of a pen, stylus or other instrument touch as distinct from afinger touch is an important attribute for many applications. In someapplications, simple detection of an instrument touch event may besufficient. Other applications may also require the ability todistinguish between different types of instruments.

Although separate mechanisms can be used to support instrument touchdetection, it is preferable for an optical touch-sensitive device to beable to provide these features with little or no hardware modification.Instruments can broadly be categorized as either passive or active.Passive instruments interact with the optical beams transmitted betweenemitters and detectors but do not add energy. Active instruments may addenergy and may contain their own emitter(s) and detector(s). Activeinstruments may be battery powered and typically will also containanother communications channel, for example a wireless connection, inorder to coordinate their operation with the rest of the optical touchdetection system. One advantage of instruments compared to fingers, isthat the instrument, and specifically its tip, can be designed toachieve a specific touch interaction with the optical beams. Differentinstruments can be designed to implement different touch interactions,and they can then be distinguished on that basis.

At least two classifications of data are available in an optical touchdetection system for detecting instrument touches: beam data and celldata. Beam data relates to the attenuation experienced by the opticalbeams in the system. It should be noted that the attenuation for somebeams under a touch can be negative (i.e., there is increased opticaltransmission) under certain circumstances, typically where there isreflection or scattering. Cell data relates to small areas (typically afew millimeters across) on the touch-sensitive surface where theattenuation values for beams passing through each of these areas isaggregated to give an indication of the localized activity in the areaof the cell. The whole touch-sensitive surface is divided into cells.

In some cases, finger, palm and instrument contacts may all be presentsimultaneously on the touch-sensitive surface. In order to discriminatebetween instrument and other contacts, and between instrument identities(there may be more than one instrument associated with the system), eachcontact area can be analyzed.

V. Passive Instrument Detection

Instruments can be designed so that their touch interactions areuniquely identifiable. Some features are more reliable than others, somemore rapidly detectable than others and some more easily implementedthan others. In practice, more than one feature may be used to provideimproved instrument detection and/or identification. Example featuresinclude: (A) contact area, (B) contact absorption rate overdistance/area, (C) contact landing behavior over time, (D) pattern ofabsorption against beam angle, (E) pattern of reflection against beamangle, (F) ratio of reflection to attenuation, (G) wavelength-selectivebehavior, (H) refractive index of contact material, (I) birefringence ofcontact material, and/or (J) re-emission of received energy. Thesefeatures can each be used to detect instrument touches and also todistinguish instrument touches from other types of touches (e.g., fingertouches) and to distinguish between different types of instruments.

A. Contact Area

The contact area may be used to distinguish an instrument touch fromanother type of touch (e.g., a finger touch or touch by a differentinstrument). Because the instrument can be designed, there are moredegrees of freedom to the contact area compared to a human finger.Contact area can be designed to differ in size and shape. Instruments,such as ordinary pens or styluses, typically have a tip that is smallerthan a human finger and can be distinguished on that basis. FIGS. 9A-9Eare top views of different types of two-dimensional contact areas forinstruments. FIG. 9A shows a contact area for an instrument with a smalltip. The size alone may be used to distinguish this instrument fromhuman fingers. FIG. 9B shows an asymmetric contact area, in this examplean elongated ellipse. The different behaviors in the x and y directionscan be used to identify this instrument, because human fingers typicallyhave a more circular contact area and so are more isotropic in theirtouch interactions. FIGS. 9C-9E show contact areas that include multipledisjoint regions. In FIG. 9C, the contact area is a 2×2 array of contactregions. In this example, optical beams, such as beam 910, that hit thecontact regions will be attenuated but optical beams, such as beam 920,that pass between the contact regions will not be affected. Movement ofthe instrument may enhance this effect, making it easier to detect. FIG.9D is a one-dimensional version of the contact area of FIG. 9C. Thisalso allows some determination of the orientation of the instrument.FIG. 9E is a version where the contact regions are not on a rectilineargrid. Because the instrument is manufactured, the individual featuressuch as the individual contact regions can be made smaller and moreprecise than the features on a human finger.

B. Attenuation Rate

Although finger touches can give rise to a range of beam attenuationvalues, there is generally a maximum realizable attenuation rate perunit distance (the length of a beam which is underneath the contact) orper unit area that a finger can achieve. Instruments can be designedusing materials and/or structures that have a significantly higher rateof attenuation. When detected, this can form a strong indication of atouch by a specially devised instrument tip.

The rate of attenuation for the size of the contact is determined basedon an estimate of the contact size to be provided. Such an estimate isavailable from analysis of the beam data associated with the contactbeing assessed (for example, by counting the number of beams in thevicinity of the contact which are showing attenuation).

Once the size (and possibly also the shape) of the contact area isknown, a geometric analysis can be used to estimate the path under thecontact for each beam in the area. From this, the length of the pathunder the contact for each affected beam can be estimated and combinedwith the attenuation data for the beams to give an indication of theloss per unit distance travelled under the touch as an alternative toloss over the contact area. Both methods can give somewhat similarresults.

Note that a measurement of attenuation rate for touch types other thaninstruments could also be useful, for example to detect when a fingermay have a contaminant on it. This could be used to enable particularsoftware or hardware mechanisms to optimize performance in the presenceof contamination.

Regarding distinguishing between multiple instrument identities,mechanical or chemical modification of the instrument tip material canenable a range of attenuation rates to be achieved. The measuredattenuation rate for an instrument touch could be used to determine theidentity of the contacting instrument.

C. Touch Interaction over Time

Instruments can also be designed to have a different temporal behaviorthan fingers or other instruments. FIG. 10A shows a finger touchprogressing in time. The finger touch at time t1 initially shows a smallcontact area, perhaps consistent in size with an instrument tip, but thecontact area typically increases in size rapidly as the finger “lands”on the waveguide, as shown at time t2 and t3. This behavior over a veryshort period of time forms another distinguishing feature allowingfinger touches to be recognized.

If an instrument is rigid, then it will make contact much more quicklyas shown in FIG. 10B. At time t1, the instrument makes contact with thewaveguide and it remains with the same contact area from then on. InFIG. 10C, the instrument is rigid and has some small amount of bouncingat time t2 before settling to its final position at time t3. All ofthese behaviors are different than the finger touch shown in FIG. 10A.Discerning different touch types typically requires not more than a fewmilliseconds.

D. Touch Interaction that Varies as a Function of Beam Direction

Finger touches are reasonably isotropic with respect to how muchattenuation they introduce as a function of the direction of theinterrupted beam. However, an intentionally structured instrument tipcan show pronounced variations in attenuation as a function of thedirection of the sensing beams. The instrument response will rotate withthe instrument tip, so the detection mechanism should be capable ofidentifying the designated response at any arbitrary orientation.Conversely, the detection process can yield information about theorientation of the instrument.

FIG. 11 is a diagram of nomenclature used to define an instrumentresponse. An origin O and a 0 angle direction, denoted by the arrow, aredefined with respect to the contact area 1110 of the instrument. Forexample, the origin O may be the center of the contact area, and the 0angle may be chosen arbitrarily. An incoming optical beam 1120 isdefined by coordinates (r,θ), where r is the offset of the path of theincoming beam 1120 relative to the origin (i.e., the length of segmentOP) and θ is the direction of propagation of the incoming beam. Anoutgoing optical beam 1130 is defined by coordinates (δr, δθ) relativeto point P on the incoming beam 1120. The propagation direction for theoutgoing optical beam 1130 is δθ relative to the incoming beam 1120, or(θ+δθ) relative to the 0 angle. The outgoing beam 1130 is also offset byδr relative to point P of the incoming beam. The offset relative to theorigin O is not necessarily (r+δr), as can be seen from FIG. 11. Onoutgoing beam with (δr=0, δθ=0) is collinear with the incoming beam,outgoing beams with (δr=0) all have paths through point P, and outgoingbeams with (δθ=0) are all parallel to the incoming beam. Theinstrument's response can then be defined by the transmission functionH(r, θ, δr, δθ), which is the strength of an outgoing beam 1130 (δr, δθ)produced by an incoming beam 1120 (r,θ). For convenience, thetransmission function can be normalized so that H=1 means that theoutgoing beam 1130 has the same strength as the incoming beam 1120.

For “ideal” fingers, the contact area 1110 is circular, and thetransmission function H exhibits some symmetries. For example, thetransmission function H is independent of incoming beam direction θ.Also, due to symmetry, H(r, δr, δθ)=H(−r, −δr, −δθ). Typically, thetransmission function H is monotonically decreasing for increasingvalues of |δr| and |δθ|, i.e., for increasing offsets and increasingangular deviations of the outgoing beam. However, instruments can bedesigned specifically to violate any of these characteristics for idealfingers. For example, consider the contact areas shown previously inFIGS. 9B-9E. None of these has a transmission function H that isindependent of incoming beam direction θ.

The complicated contact area shapes in FIGS. 9B-9E can be implemented bycreating an instrument tip where the individual contact regions couplelight out of the waveguide of the touch-sensitive device (frustratedTIR) and the non-contact regions do not. For example, the contactregions may be constructed from a transparent material with an index ofrefraction that matches the waveguide, so that light propagating in thewaveguide passes into the material and then is absorbed or redirectedelsewhere. The non-contact regions may be constructed of reflectivematerials to confine light to the underlying waveguide, or may beconstructed with an air gap so that total internal reflection is notfrustrated.

FIG. 12A is a front view of a more complicated tip structure. This tipstructure includes a number of parallel waveguide channels 1210. FIG.12B is a side cross-sectional view through the center of a waveguidechannel 1210. Each waveguide channel 1210 is a strip of transparentmaterial 1212 capped by an air gap 1214. The transparent material 1212in this example has a matching refractive index to the waveguide of thetouch-sensitive device. Materials of other indices of refraction canalso be used. A higher index of refraction can shorten the traveldistance of beams within the tip, thus allowing the tip to be madesmaller. The rest of the tip is constructed from a material 1216. Thematerial 1216 can be reflective or absorptive, which will producedifferent but distinctive instrument responses.

This tip offers selective reflection behavior that depends on thedirection of the incident beam. FIG. 12C shows operation of a singlewaveguide channel 1210. Light ray 1220 is travelling along the +ydirection, parallel to the waveguide channel 1210 which is also orientedalong the y direction. As a result, light 1220 couples into thewaveguide channel 1210. In FIG. 12C, the ray 1220 is shown as dashedwhen traveling in the underlying waveguide 1250 and solid when travelingin the waveguide channel 1210. The circle indicates the point of entryor exit from the waveguide channel 1210. The light 1220 experiencestotal internal reflection within the waveguide channel 1210 beforecoupling back to the underlying waveguide 1250.

Light rays that are propagating off-parallel relative to the orientationof the waveguide channel 1210 (e.g., at small angles relative to the ydirection) will couple less efficiently. For example, a beam travelingalong the x direction (perpendicular to the waveguide channel) may enterone or more of the waveguide channels, but will strike the side wall ofthe channel because the channels are not as wide as they are long. Thatray will either be absorbed or reflected in large part by that sidewall, depending on whether the side wall material is absorptive orreflective. This different behavior for rays propagating in the x and ydirections yields a distinctive instrument response. This tip acts as asort of directional filter, since the transmission function H(r, θ, δr,δθ) is concentrated along certain preferred directions θ.

In addition to designing different patterns of attenuation, instrumentscan also be constructed to redirect incident light on paths other thanthose which would occur by propagation through the waveguide. Aninstrument tip of this type typically will include some reflectiveelements. As a result of such an instrument tip being in contact withthe sensing waveguide, optical transmission will be reduced on someoptical paths and increased on others. This is not a pattern that wouldnormally occur with fingers, for example.

FIG. 13 is an example of an instrument tip that uses a prism 1310 toredirect light. In FIG. 13, the prism 1310 is represented by its base1312 and two surfaces 1314 and 1316. The full prism is not drawn in FIG.13 for clarity. In this example, the prism 1310 is constructed of atransparent material that is index matched to the underlying waveguide1350 (although index matching is not required). Light ray 1320 ispropagating along the +y direction in the waveguide and couples into theprism 1310 through its base 1312. The ray reflects off the two sides1314, 1316, which are coated with reflective material. In some designs,the reflection may be the result of total internal reflection. Theexiting light ray 1322 is redirected to the +x direction and at an anglethat supports total internal reflection within the waveguide 1350. There-routing of light can also be accomplished using other elements, suchas retroreflectors, fibers, light pipes or waveguides. Gratings or otherbeamsplitting elements can be used to create multiple exit opticalbeams.

The transmission function H can be used in different ways to identifyinstrument touches and to distinguish different instruments. Forexample, many transmission functions are characterized by certaindirections that exhibit strong attenuation or strong enhancement. Thatcharacteristic can be used to detect and identify instruments. The ratioof attenuated and enhanced beams can also be used. If there are multipleoutput beams, the number of beams with greater than a certain strengthcould be used.

As a further variation, the transmission function for an instrument mayalso depend on the orientation of the instrument. The instrument tip inFIG. 12 has a flat bottom, which is intended to be flush against thetouch-sensitive surface. If the instrument is tilted, the bottom willnot make the same contact and the transmission behavior will bedifferent. Mathematically, the transmission function can be described asH(r, θ, δr, δθ, α, β) where a, β define the orientation of theinstrument relative to the touch-sensitive surface. In some instruments,the transmission function is intended to be independent of theorientation α, β. Instruments with rounded tips help to facilitate thisbehavior because at least the physical contact will be the same—arounded tip against a flat touch-sensitive surface—regardless of theorientation of the instrument. Alternatively, the instrument can beintentionally designed to have a transmission function H(r, θ, δr, δθ,α, β) that varies as a function of the instrument orientation α, β.

Regarding distinguishing between multiple instruments, since many beamsare affected by an instrument touch, this allow the transmissionfunctions of different instruments to contain unique features to beidentified. The processing workload associated with instrumentidentification will be strongly dependent on the number of differentinstruments to be identified. Also, other touches may occursimultaneously with one or more instrument touches, so reasonablycomplex transmission functions are preferred to provide robustidentification.

E. Wavelength-Selective Touch Interaction

Wavelength can be used to add another dimension to touch interactions.This can allow touches to be assessed in a way which will readilydistinguish an instrument tip which has spectral properties, such asnarrowband absorption or reflection properties. Different wavelengthscan be implemented at the emitter by using different emitter types or byselective use of optical filtering materials in the emitter couplers (tomodify the limited spectrum generated by a single broadband LED type).Detectors are typically sensitive over a wide range of wavelengths, soenergy from various emitter wavelengths can be detected. Note that theproportion of emitters operating at one wavelength relative to thoseoperating at other wavelengths could be small or additional emittersprovided specifically to aid with identification. An extension of thisscheme would be to use emitters with visible wavelengths to detect thecolor of the contacting material. This kind of color detection could,for example, be used in drawing applications, where the color of thecontacting material could be applied to the path traced by the contact.Alternately, broadband emitters could be used, with wavelengthselectivity implemented at the detector. Detectors sensitive atdifferent wavelengths could be used, or optical filters could be usedwith broader band detectors.

Regarding distinguishing between different instruments, in oneimplementation, one instrument might absorb (e.g., cause attenuation dueto frustrated TIR) at a first wavelength but not at a second wavelength,while a different instrument absorbs at the second wavelength but notthe first. Alternatively, instruments could be distinguished based onratios of attenuation at different wavelengths. This could be extendedto more than just two wavelengths.

FIG. 14 is a diagram of an instrument tip that attenuates over a narrowwavelength band. The tip includes transparent material 1410 with amatching index of refraction to the underlying waveguide 1450 (althoughindex matching is not required). The tip also includes a narrowbandspectral filter 1412, which passes wavelengths in a narrow passbandcentered at λ₀. Light ray 1420 is at wavelength λ₀, passes through thewavelength filter 1412, enters material 1410 and then is absorbed orotherwise prevented from reentering the waveguide 1450. Light ray 1422is at a wavelength λ₀ that is outside the passband. This ray 1422 isreflected by the wavelength filter 1412 and remains in the waveguide1450. In an alternate approach, the wavelength filter 1412 could block awavelength band rather than transmitting a wavelength band.

FIG. 15 is a diagram of an instrument tip 1510 that uses a grating 1512.Light ray 1520 is at a shorter wavelength and light ray 1522 is at alonger wavelength. Both rays are diffracted into the first diffractionorder. However, the angle of diffraction is greater for the light ray1522 of longer wavelength. The diffracted ray 1520 still propagates atan incidence angle that is beyond the critical angle and remainsconfined within the waveguide 1550. However, the diffracted ray 1522 nowpropagates at an incidence angle that is less than the critical angle.Total internal reflection is lost and the optical beam 1522 isattenuated. The angle of diffraction and resulting wavelength behaviorcan be varied by changing the period of the grating and/or theefficiency of coupling into different diffraction orders.

F. Index of Refraction, Birefringence

Instruments can be constructed using materials with different indices ofrefraction, including birefringent materials. They can then bedistinguished on this basis. The critical angle at an interface dependson the indices of refraction of the materials on both sides of theinterface. Changing the index of refraction of the instrument tipchanges the critical angle, which in turn affects whether an opticalbeam is transmitted into the instrument tip (i.e., removed from thewaveguide) or total internally reflected back into the waveguide. If theoptical beam includes a distribution of rays at different angles ofincidence, then some may be transmitted and some reflected so that theaggregate attenuation of the instrument is between 0 and 1. Differentangles of incidence can be provided by different coupler profiles.Synthetic materials can be produced with a very wide range of refractiveindices.

FIG. 16 is a diagram of a tip 1610 with an index of refraction betweenair and that of the underlying waveguide 1650. The incoming beam 1620includes rays propagating across a distribution of propagation angles.For clarity, FIG. 16 shows only four angles, two of which are shown bysolid lines and two by dashed lines. All of these rays are beyond thecritical angle so they experience total internal reflection at sectionsof the waveguide 1650 which border air (i.e., where there is no touch).The instrument 1610 has a higher index of refraction so some of thesteeper rays (dashed lines) couple into the instrument, which appears asattenuation at the detectors. Shallower rays (solid lines) are stillbeyond the critical angle for the interface with the instrument, so theyare reflected at the interface and continue propagating within thewaveguide 1650. By changing the index of refraction, the percentage ofrays transmitted at the interface and the overall attenuation can beadjusted. Birefringent materials can be used to construct instrumentswith even more complex transmission functions, since the index ofrefraction varies as a function of polarization and angle ofpropagation.

G. Re-Emission of Light

Photoluminescence is not present for finger touches but can be achievedby some synthetic materials. Fluorescence involves a spectral shiftbetween the received and emitted energy. Phosphorescence does notnecessarily have a spectral shift, but does introduce a time lag betweenreception and emission and may also include a change in propagationdirection. Detection of the temporal “smearing” of the sensing energywould be readily detectable. Different chemistries can also providewidely different time constants.

VI. Active Instrument Detection

Actively powered instruments (active instruments) can provide additionalcapabilities. An active instrument may use solely optical input/outputto operate in conjunction with the optical touch-sensitive device, ormay have a wireless or other communications link with which tocommunicate data to the touch-sensitive device. The instrument tip mayinclude optical emitter(s) for light injection into the underlyingwaveguide, optical detector(s) for light extraction from the waveguide,or both optical detector(s) and optical emitter(s). Instruments withemitter(s) will be referred to as injector instruments and those withdetector(s) will be referred to as extractor instruments.

An active instrument can provide advantages, including possibly thefollowing. (1) An active instrument can be designed to support differentmodes of operation. In addition, the mode might be selectable on theinstrument itself (for example, the instrument color). (2) Adding activefunctionality increases the number of possible designs, thus allowing alarger number of possible instrument identities to be distinguished. (3)Additional buttons and other user controls can be added to activeinstruments. (4) Force measurement and reporting can be added. (5) Theinstrument orientation, position, movement, etc. can be sensed andreported, for example by using accelerometers and gyroscopes. This canhelp improve the overall touch performance, especially when theinstrument is moving fast. (6) Wireless connections can be implemented,which in turn can enable additional functions. For example, non-contactoperations can be provided. Improved instrument tracking can beaccomplished using supplementary data. Or advanced notice of imminentinstrument activity can be provided.

Active instruments require a source of power. Batteries are one option,either replaceable batteries or rechargeable cells. Recharging might bedone when the instrument is at rest in a holder.

A. Injector Instrument

FIGS. 17A-17B are a side view and top view of an injector instrument1710. The injector instrument 1710 houses an optical emitter 1712 thatinjects (modulated) light 1720 into the waveguide 1750, and that light1720 can be detected by the detectors of the touch-sensitive system. InFIG. 17B, the instrument 1710 produces only a single optical beam 1720.The light 1720 from the instrument can be designed so that it isdistinguishable from the optical beams produced by the emitters on theperiphery of the device, for example by using different time slots,wavelengths and/or modulation. The optical beam 1720 from the instrumentcan be used for different purposes.

For example, the optical beam 1720 can be used as a communicationschannel and not at all for touch detection, which may be accomplishedusing optical beams as described above. The communicated data caninclude any information about the instrument: its identity, operationalmode or operational parameters, contact force, position, orientation ormotion, for example. The data can be encoded using standard methods,such as modulating the optical beam 1720. A single beam 1720 issufficient as long as the beam can be detected by any detector on theperiphery. If the position of the instrument is known by other means,then which detector(s) receives the optical beam 1720 can be used todetermine the orientation (rotation) of the instrument.

The optical beam 1720 can also be used for touch detection. In FIG. 17B,if the single optical beam 1720 has characteristics that vary as afunction of distance, for example if it is a fan-shaped beam so that thefraction of the beam intercepted by a detector decreases when thedetector is farther away, then this can be used to assist in determiningthe touch location of the instrument.

In FIG. 17C. the instrument produces three fan-shaped beams 1720A-C.Distance from the instrument 1710 can be estimated based on the strength(or relative strengths) of the signals received by the detectors. Thiscan then be used to triangulate the position of the instrument 1710. InFIG. 17D, the instrument produces four pencil beams 1720A-D, defining xand y directions relative to the instrument. The x-axis for theinstrument can be estimated by a line connecting the detectors receivingoptical beams 1720B and D, and the y-axis estimated by a line connectingthe detectors receiving optical beams 1720A and C. The intersection ofthe x and y axes determines the touch location of the instrument 1710.These techniques can also be combined with the prior describedtechniques for determining touch events based on disturbing opticalbeams transmitted between emitters and detectors.

Regarding instrument identity, different instruments can be identifiedby having them emit different optical beams. Optical beams can usedifferent wavelengths, time slots, frequency bands, encodings ormodulations, etc. These can be used to distinguish differentinstruments.

B. Extractor Instrument

An instrument with a detector which detects optical beams from theunderlying waveguide can implement various functions. First, the opticalbeam received can be used as a communications channel to transmit datafrom the rest of the touch-sensitive system to the instrument. In thiscase, the optical beam may or may not be one of the optical beamsnormally used for touch detection. In one approach, the optical beam isbroadcast over a large area so that the instrument detector will receivethe beam even if the position of the instrument is not known. In anotherapproach, the position of the instrument is known and the optical beamis directed to that position.

An extractor instrument may also be used in touch detection. Forexample, the detector may be used to detect which optical beams fromperiphery emitters are received by the instrument. This information maybe used to directly determine the position of the instrument, ratherthan or in addition to the prior described techniques for determiningtouch events based on disturbing optical beams transmitted betweenemitters and detectors.

C. Bidirectional Instruments

An instrument may contain both an emitter and a detector, in which caseit is both an injector instrument and an extractor instrument. Theseinstruments will be referred to as bidirectional instruments. There ismore than one way in which such an instrument could interact with thetouch-sensitive system. For example, it could simulate photoluminescenceby emitting a delayed version of the signal received at the detector.Alternatively, the detector can be used to synchronize the emitteractivity with that of the rest of the touch-sensitive system. Theemitter and detector can also be used as a bidirectional communicationchannel for the transmission of data to/from the rest of thetouch-sensitive system.

Regarding instrument identity, the coincident appearance of theinstrument and the associated emitter modulation can provide informationabout the instrument identity. Also, the specific signal seen by theinstrument detector can provide additional information regarding theapproximate location of the instrument. Additional mode information frombuttons and the like on the instrument can also be passed to the rest ofthe system.

D. Out of Band Communication Channel

Active instruments may have communication channels other than throughthe touch interaction, most likely wireless channels. The use of awireless link provides ready support for many other features, such asmode selection and presentation control when not in contact with thetouch-sensitive surface. Also, supplementary data from accelerometers,gyroscopes and other sensor types can be sent, which can be combinedwith the optically resolved tip location to provide improved sensing ofthe instrument motion.

Since accelerometers and gyroscopes are often relative sensors whichtend to drift over time, the combination with an absolute determinationby the optical waveguide-based touch sensing is a powerful one. Aparticular attribute of an active instrument using accelerometers isthat the rate of movement which can be handled is much increased. Theaccelerometer data can inform the optical waveguide sensor as to wherethe instrument tip is likely to be found in the next scan. This helps tocompensate for motion blur.

Relating the specific movement of an instrument as reported frominternal motion sensors to the movement seen on the optical waveguidesensor is a possible way of confirming the instrument identity. Thesensors could be internal to a specially constructed instrument or couldbe in a “collar” attached to a passive instrument. For example, anordinary whiteboard marker will typically register well as a passivecontact on an optical waveguide touch system. This can be useful inapplications in which there is no graphical display associated with thetouch-sensitive surface, but the path traced by an instrument on thesurface is to be determined. However, attributes of the marker, such asthe color of the ink, may not be easily detected by the waveguide-basedsystem. Supplementary electronics attached to the marker could transmitto the touch-sensitive system data (for example, data relating to themotion of the instrument) which could be matched by the system with acorresponding path on the touch-sensitive surface. Once the sensed touchhas been matched to the instrument, then attributes known by the systemcan be applied to the reports associated with that instrument. Oneexample is the color of the marker. Another example is the size of themarker tip. Using motion information for identification and tosupplement the quality of the reported motion could be applied toobjects other than instruments, including fingers and objects which areto be used as physical controllers in contact with the touch-sensitivesurface.

E. Power Management for Active Instruments

Regarding instrument charging, the body of a typical large instrument isreasonably well-suited to the use of popular cylindrical cells which maybe non-rechargeable or externally rechargeable. However, it is likely tobe preferable for the instruments to be rechargeable and for thecharging to be supported by the touch-enabled device. An instrumentholder could be provided with a facility to recharge the batteries, forexample.

Regarding a low power mode, removal of an instrument from the holdercould trigger the circuitry inside the instrument to be ready forpresentation to the touch-sensitive surface. Otherwise, the instrumentwould be charging or in standby (low power) mode. Instruments withsupplementary internal motion sensing (such as accelerometers) can usemotion detection to control the internal circuit activity. When theinstrument is motionless for a period of time, operation can besuspended. Occasional checking of the motion sensors can ensure that theinstrument is fully operational when it comes into contact with thetouch-sensitive surface.

When such an instrument is in motion and/or determined to be inproximity to the touch-sensitive surface, the touch-sensitive system canbe placed into a mode which increases the time over which finger touchesare analyzed before being reported. This is beneficial to reduce thechances of a spurious finger report being generated as the side of thewriting hand lands on the touch-sensitive surface. The side of a handcan generate a finger-sized contact before coming fully to rest on thetouch-sensitive surface. In this mode, the sensor will be slightlyslower to respond to finger touches, but that will often be acceptable.

VII. Additional Modalities

A. Palm Management

When instruments are used with touch-sensitive surfaces more than a fewinches in dimension, the user is likely to rest the side of the writinghand on the touch-sensitive surface. These are commonly known as “palm”touches even though they are most often associated with the side of thehand rather than the palm.

An optical waveguide touch-sensitive system can accommodate palm touchessince not all of the light passing under the palm is likely to beabsorbed and the pattern of optical beams can be arranged so that thereis a high likelihood of some passing under the instrument tip, but notunder the palm. For example, with optical emitters and detectors aroundthe periphery of the touch-sensitive surface, there will usually beoptical beams travelling between top and bottom (or front and back) ofthe surface which pass through the instrument contact area withouttouching the palm contact area, which will usually be to the right orleft of the instrument tip.

The first step in managing palm touches is to detect them. One way todetect a palm touch as distinct from an instrument touch is to dividethe touch-sensitive surface into regions which can be referred to as“cells.” In one embodiment, possible instrument touches are qualified,including eliminating palm touches, using cells according to the methodshown in FIG. 18.

Divide 1810 the touch-sensitive surface into regions (cells) which aresmaller than a palm's contact area and larger than an instrument'scontact area. Determine 1812 which beams pass through each cell.Calculate 1814 the proportion of beams in the cell which have beendisturbed by a touch. If the proportion is large 1820, then reject thecell as a possible instrument touch. If the proportion is small (e.g.,below a threshold percentage) 1820, it may be an instrument touch but itmay also be part of a larger palm touch. Consider these cells to becandidate instrument cells. For the candidate instrument cells, check1822 the neighboring cells. Candidate instrument cells which haveneighboring cells that show beam activity consistent with a palm touchare rejected as instrument touches, on the basis that the candidateinstrument cell is probably a cell at the edge of a palm touch. In theremaining candidate instrument cells, calculate 1825 the range of anglesof active beams. If the angular range is small 1830, the beam activityin the cell may be an artifact of touches elsewhere and the cell isrejected as an instrument touch. Otherwise 1830, the cell remains 1832 aviable candidate for an instrument touch. Other techniques can beapplied to further determine whether there is an instrument touch.

Once palm activity has been identified, some applications may reject itby doing nothing more with the palm information. Other applications maydetermine attributes of each palm touch, such as the location, shape andsize. Information about a palm touch can be used to provide enhancedperformance. When an instrument is being used and the side of thewriting hand is detected on the touch-sensitive surface, all furthertouches between the palm and the instrument tip can be ignored. Forexample, finger-like touches on the instrument side of a palm are likelyto be associated with inadvertent finger/knuckle touches with thetouch-sensitive surface. Some knowledge of the location and extent ofthe palm touch is helpful in determining the region in which inadvertenttouches are to be ignored. A simple approach would be to ignorefinger-like touches in a region of fixed size around the instrument tip.

B. Acoustics

Mechanical vibrations at the surface of a touch-sensitive device can beuseful to determine the nature of a material that touches the surface.For example, a hard material landing on the surface will typicallygenerate a sharp acoustic transient. Augmenting information from awaveguide touch-sensitive device with acoustic input from a contactmicrophone or other transducer associated with the touch-sensitivesurface can provide additional capabilities to identify materials.

When the touch-sensitive device detects a new touch at a time which iscoincident (within margins which allow for the respective latency of thetwo sensing methods) with the acoustic report, the acoustic signal canbe associated with that touch. Further confidence in that associationcan be gained by analysis of the acoustic signal generated by movementof that touch and its consistency with the activity detected by thetouch sensor.

An example of the application of such a system is the detection ofinstrument touches as distinct from finger touches on a touch-sensitivesurface. If the instrument tip is composed of material which is notsimilar to a finger, then the characteristics of the vibration patterngenerated on landing will be different. The tip material is important,but also the composition of the instrument itself. An instrument with alarge mass will generate a different landing transient than one which islightweight. Appendages may be introduced into the design of aninstrument specifically to provide a distinctive acoustic signature. Forexample, a loose mass in the hollow body of an instrument can give riseto a second transient after the one generated by the landing of the tip.

The acoustic signal can also be used to reduce the power consumption ofa touch-sensitive system. Scanning of the system may be disabled untilan acoustic signal is detected, which may indicate the arrival of a newtouch on the touch-sensitive surface.

Also, vibrations are typically present when a touch is lifted from thesurface. This information can be particularly useful in overcoming the“stuck” touch problem in optical waveguide systems, where a patch of acontaminant left by a touch which has been lifted is sufficient for thesystem to falsely report that the touch is still present. The acousticsignal associated with the touch being lifted provides a helpful cluethat it has in fact been removed and that only contamination remains onthe surface at that location.

Acoustic detection can robustly reject vibrations caused by unrelatedvibration activity. This can be achieved firstly by relying onhigh-frequency components in the acoustic signal. These are usuallyabsorbed rapidly when passing through the body of a device. For example,an acoustic sensor in a computer display monitor on a desk will notusually receive much high frequency vibration energy from the deskbecause the desk material, the monitor housing and the soft padding onthe underside of the monitor base will absorb it. So, the high frequencyenergy (which is also the energy that results in fast-moving acoustictransients) will usually result only from vibrations introduced at thesurface of the monitor itself. Also, more than one acoustic sensor canbe used and analysis of the signals from multiple sensors can determinewhether the vibration originated at a point which is outside of thetouch-sensitive area. For example, where there are contact microphonesat the left and right edges of the touch-sensitive area, a transientgenerated by a landing event on the touch-sensitive area should arriveat the two sensors with a time difference which is smaller than thetime-of-flight of the vibration across the area. If the time of arrivalfor the signal at the two sensors is different by the full span of thesurface between them, then it can be concluded that it originated at apoint which is outside of the touch-sensitive surface.

An extension of this time-of-flight analysis of the acoustic signal candetermine an approximate location to be compared with that reported bythe optical waveguide touch sensing so that the association between themcan be more reliable.

Analysis of the signal can also be performed to determine the rise timeand/or frequency spectrum of the transients and vibrations. Afast-moving transient edge can be identified by directly measuring therate at which the signal changes over a succession of time-spacedsamples, or by comparing the sampled signal with a synthetic or recordedtemplate of a transient. Fast-moving/high frequency energy can also bedetected by looking at the frequency content (spectrum) of the signal.In terms of Fourier synthesis, high harmonics have significant magnitudein such rapid acoustic transients. Again, this characteristic can bedetermined by directly analyzing the spectral content, or by comparisonof the spectrum with a pre-determined template spectrum.

This type of analysis can also reveal information about the speed ofmovement of the touch along the touch-sensitive surface, particularly ifthere is some profiling of the touch-sensitive surface (although this isnot a requirement). Estimation of the speed of travel can be done byanalysis of the signal intensity, phase or spectrum at many transducers,or by the spectrum or change of intensity, phase or spectrum at a singletransducer. A speed estimate can be useful supplementary data for atouch-sensitive sensor since it can help to estimate an expectedlocation from one scan to the next.

When a touch is moving quickly, successive reported locations from thetouch-sensitive system may be quite far apart, and it may not be obviousthat the reports relate to the same touch. If the speed is known to behigh, based on the acoustic signal, then this relationship betweensuccessive reports can be established.

Different touch types, or specific instances of touch types can bediscerned using acoustic sensors. For example, two instruments could bedistinguished by virtue of the tip material, the instrument mass or byadditional aspects of the instrument design which causes the vibrationsin the touch-sensitive surface to be distinguishable.

Contact microphones and similar transducers can be attached to theunderside of the touch-sensitive surface (i.e. the side facing away fromthe user) so that an uncluttered surface can be presented to the user.These would be connected to analog-to-digital conversion circuitry andthe resulting time-sampled data made available to a microprocessorsystem. Some or all of the analysis could optionally be performed usinganalog electronics, but that is likely to be less preferable thandigital processing.

VIII. Additional Considerations

The figures depict embodiments of the present invention for purposes ofillustration only. One skilled in the art will readily recognize fromthe following discussion that alternative embodiments of the structuresand methods illustrated herein may be employed without departing fromthe principles of the invention described herein.

Upon reading this disclosure, those of skill in the art will appreciatestill additional alternative structural and functional designs throughthe disclosed principles herein. Thus, while particular embodiments andapplications have been illustrated and described, it is to be understoodthat the disclosed embodiments are not limited to the preciseconstruction and components disclosed herein. Various modifications,changes and variations, which will be apparent to those skilled in theart, may be made in the arrangement, operation and details of the methodand apparatus disclosed herein.

What is claimed is:
 1. A method for detecting an instrument touch eventon a touch-sensitive surface, the surface having emitters and detectorsarranged around its periphery, the emitters producing optical beamsreceived by the detectors, the instrument touch event disturbing theoptical beams, the method comprising: receiving information indicatingwhich optical beams have been disturbed; analyzing the receivedinformation to detect one or more touch events; and based on saidanalysis, identifying at least one of the touch events as an instrumenttouch event rather than a finger touch event.
 2. The method of claim 1wherein the optical beams propagate from emitter to detector by totalinternal reflection, and optical beams are disturbed as a result offrustrated total internal reflection caused by touches on thetouch-sensitive surface.
 3. The method of claim 1 wherein the instrumenttouch event is distinguished from finger touch events based on a contactarea of the instrument touch event.
 4. The method of claim 3 wherein theinstrument touch event is distinguished from finger touch events basedon the instrument touch event having a contact area containing two ormore disjoint regions.
 5. The method of claim 1 wherein the instrumenttouch event is distinguished from finger touch events based on theinstrument touch event having a higher beam attenuation rate than fingertouch events.
 6. The method of claim 1 wherein the instrument touchevent is distinguished from finger touch events based on a temporalbehavior of a touch interaction for the instrument touch event.
 7. Themethod of claim 6 wherein the instrument touch event is distinguishedfrom finger touch events based on the touch interaction for theinstrument touch event including bouncing at the touch-sensitivesurface.
 8. The method of claim 1 wherein the instrument touch event isdistinguished from finger touch events based on a transmission functionfor the instrument, wherein the transmission function expressesstrengths of outgoing optical beams as a function of strengths ofincoming optical beams.
 9. The method of claim 8 wherein the instrumenttouch event is distinguished from finger touch events based on theinstrument's transmission function varying as a function of directionsfor the incoming optical beams.
 10. The method of claim 8 wherein theinstrument touch event is distinguished from finger touch events basedon the instrument's transmission function acting as a directional filterfor the incoming optical beams.
 11. The method of claim 8 wherein theinstrument touch event is distinguished from finger touch events basedon the instrument's transmission function acting to redirect predefinedincoming optical beams in a predefined manner.
 12. The method of claim 8wherein the instrument touch event is distinguished from finger touchevents based on the instrument's transmission function acting to split apredefined incoming optical beam into multiple outgoing optical beams ina predefined manner.
 13. The method of claim 8 further comprising: basedon said analysis, determining an orientation of the instrument.
 14. Themethod of claim 1 wherein the instrument touch event is distinguishedfrom finger touch events based on a wavelength behavior of a touchinteraction for the instrument touch event.
 15. The method of claim 1wherein the instrument touch event is distinguished from finger touchevents based on a behavior of a touch interaction for the instrumenttouch event resulting from an index of refraction of material in a tipof the instrument, wherein the material in the instrument tip isbirefringent.
 16. The method of claim 1 wherein the instrument touchevent is distinguished from finger touch events based on a re-emissionof optical beams by the instrument.
 17. The method of claim 1 furthercomprising: analyzing the received information to detect a palm touch;and based on said analysis, eliminating the palm touch as possibleinstrument touch.
 18. The method of claim 17 wherein the instrumenttouch event is distinguished from finger touch events based on the palmtouch occurring in a vicinity of the instrument touch event.
 19. Themethod of claim 1 further comprising: receiving acoustic informationrelating to contacts with the touch-sensitive surface; and analyzing thereceived acoustic information, wherein the instrument touch event isdistinguished from finger touch events based on the analyzed acousticinformation.
 20. The method of claim 19 wherein the instrument touchevent is distinguished from finger touch events based on a highfrequency content of the received acoustic information.
 21. An opticaltouch-sensitive device comprising: a surface for which touch events areto be detected; emitters and detectors arranged around a periphery ofthe surface, the emitters producing optical beams received by thedetectors, the touch events disturbing the optical beam; and a touchevent processor coupled, directly or indirectly, to the emitters anddetectors, the touch event processor receiving information indicatingwhich beams have been disturbed by actual touch events; analyzing thereceived information to detect one or more touch events; and, based onsaid analysis, identifying at least one of the touch events as aninstrument touch event rather than a finger touch event.